How P6 works with currencies ?

Q1. What happens with the current projects costs if the base currency changes?

Q2. How can I 'freeze' the project costs in budgeted currency from the past for future analysis?

For example, I have US$ as base currency and a part of projects budgeted in Euro (view currency with exch rate at year 2010).

If I update the exch rate for Euro at year 2015, I expect the project costs budgeted in Euro in 2010 to change based on new exch rate.

I have a feeling that is not possible to keep the original budget in Euro.
What happens if I change now the base currency in Euro? The original US$ will be recalculated?